1. Field of the Invention
The present invention relates to a magnetic resonance antenna for nuclear magnetic resonance systems with a vertical basic field, the magnetic resonance antenna being of the type having a number of antenna elements that extend substantially radially from an inner element to at least one outer element, relative to a center axis, the antenna elements being at least magnetically coupled with one another.
2. Description of the Prior Art
A magnetic resonance antenna of the above type is known from U.S. Pat. No. 5,153,517. This magnetic resonance antenna has four antenna elements that are driven in pairs with 90xc2x0 phase quadrature.
So-called birdcage resonators are known for magnetic resonance systems with an axial basic field. These have a number of antenna elements that are circularly arranged around a center axis and that extend parallel to the center axis. These antenna elements, at their ends, are connected to one another via ring-shaped connecting elements in an electrically conducting manner.
An object of the present invention is to provide a magnetic resonance antenna for magnetic resonance systems with a vertical basic field, with which a homogeneous circularly polarized magnetic field can be generated in a simple manner.
The above object is achieved in accordance with the principals of the present invention in a magnetic resonance antenna formed by a number of antenna elements which proceed substantially radially from an inner element beginning to at least one outer element and, relative to a center axis, the antenna elements being at least magnetically coupled with one another, and wherein the number of antenna elements is at least five.
The antenna elements are coupled with one another at least via their magnetic field. In addition, they can also be coupled with one another via connecting elements at the element beginnings and/or element ends. Such an electrically conducting connection is preferably effected via a connecting element that is fashioned in a ring-shaped manner. Alternatively, the element beginnings and/or element ends can be individually connected to a grounding plane instead of being connected among each other with connecting elements.
Capacitors can be arranged in all antenna elements and/or connecting elements in order to adapt the magnetic resonance antenna to the desired operating frequency.
Regardless of the type of coupling, what is important in both cases is that the antenna elements are coupled with one another such that the magnetic field, which is generated by them, is circularly polarized.
In the most simple case, the antenna elements can each have one element beginning and one element end. As warranted, however the antenna elements can be split toward the outside, i.e. they can have at least two element ends.
The geometry of the magnetic resonance antenna can be even. The construction of the magnetic resonance antenna is particularly simple in this case. When the element beginnings define an element beginning plane and the element ends a element end plane, and when the element beginning plane and the element end plane extend parallel to one another and are offset to one another, a particularly homogenous magnetic field results in the area of the center axis. Preferably, the antenna elements are fashioned linear in this case.
The antenna elements each exhibit a line direction, whose extrapolation cuts the center axis in a common intersecting point. The magnetic field generated by means of the magnetic resonance antenna is more homogenous when the element beginning plane and the element end plane extend parallel to a grounding plate. It is particularly homogenous when the intersecting point is situated in the grounding plate.
When the number of antenna elements can be divided by four, the coupling-in and coupling-out of two currents that are phase-shifted by 90xc2x0 is particularly simple.